Use a fence post auger by determining the location of the holes, manually digging a shallow pilot hole, and placing the auger in the hole. Start the motor and turn the throttle to dig into the ground, then stop and lift the auger out of the ground to clear the dirt. Unless using a small one-man auger, enlist a strong helper to work with you on the project.
If digging more than five postholes, using a power auger makes sense; otherwise, dig the holes manually. Augers are available at tool rental stores. There is often a choice in available bit sizes, but larger bits make the machine more difficult to operate. Choose the size appropriate for the job you are doing. A 12-inch auger leaves room for pouring cement around the post, but a 4-inch one only works with very small posts.
Prior to using the machine, call the national Call Before You Dig center to have the service locate all utilities in the area. When using the machine, wear safety glasses, hearing protection, sturdy clothing and boots. The job also requires a manual clamshell posthole digger, spade and a large breaker bar. Approximately one in four holes require digging by hand due to obstructions in the ground.